Plasma arc cutting torches have a wide variety of uses, such as the cutting of aluminum sheet metal, thick plates of steel or stainless steel, or thin sheets of galvanized metal. As illustrated in FIG. 1, a plasma torch 10 includes a torch body 12, an electrode 14 and a nozzle 16 mounted within the body. The nozzle has a central exit orifice 18 and can be surrounded by a shield 22. An exit port 24 of the shield is generally aligned with the exit orifice 18 of the nozzle. A power supply (not shown) is used to create an arc between the electrode 14 and the nozzle 16 ionizing a plasma gas that is supplied from a plasma gas source 30. The ionized plasma gas exits the torch 10 through the exit orifice 18 of the nozzle and the exit port 24 of the shield, and is used to cut a workpiece (not shown). Once the plasma arc has been initiated, the current flow can be transferred from the nozzle to the workpiece.
The shield 22 is mounted to a retaining cap 26 on the torch body 12. Shield gas from a shield gas source 40 can be introduced to a space between the nozzle 16 and the shield 22. At least a portion of the shield gas exits the torch with the plasma arc (i.e., the ionized plasma gas) through the exit port 24 of the shield. The shield gas cools the shield and helps protect the shield from workpiece splatter during a cutting or piercing operation of the torch. The torch can include a swirl ring (not shown) in the flow path of the plasma gas and/or the shield gas to impart a swirling motion to the gas for improving torch performance.
During operation of the torch, certain consumable parts become worn and have to be replaced. These consumables include torch electrodes, nozzles, and shields. Previous patents assigned to Hypertherm, Inc. of Hanover, N.H. teach techniques for prolonging the life of these consumables. For example, U.S. Pat. No. 5,070,227, the contents of which are incorporated herein by reference, teaches that the life of an electrode can be extended by controlled reduction of the plasma gas flow a short time before commencement of the torch current flow reduction, as the cut cycle is ended. U.S. Pat. No. 5,166,494, the contents of which are incorporated herein by reference, describes altering the flow of plasma gas in conjunction with the transfer of the current flow from the nozzle to the workpiece, and U.S. Pat. No. 5,170,033, the contents of which are incorporated herein by reference, explains that a chamber in the swirl ring can be created and sized to favorably affect the dynamic flow characteristics of the flowing gas when torch operating conditions are changed.
FIG. 1 illustrates a known gas distribution feed system for a plasma arc torch. Gas from a gas supply system or a gas cylinder (e.g., plasma gas supply 30 or shield gas supply 40) is regulated to a desired operating pressure using a pressure regulator 31, 41. The reduced-pressure gas passes through an on-off solenoid valve 33, 43 and optionally through a manually operated needle valve 35, 45. On-off solenoid valves (e.g., 33, 43) generally produce an exponentially decreasing pressure decay curve upon closure of the valve. After exiting the needle valve 35, 45, the gas flows to a plasma torch 10. The plasma gas is channeled to the plasma chamber, to a space located between the electrode and the nozzle. The shield gas flows to a space between the shield and the nozzle. A more complex gas distribution feed system is described in U.S. Pat. No. 5,396,043, assigned to Hypertherm, Inc., the contents of which are incorporated herein by reference. The '043 patent describes a plurality of plasma and/or shield gas flow channels and valves of different sizes and configurations that can be used to provide a pre-flow, an operating flow, and a quick charge for use in different operating modes of the torch, such as during piercing operation of the torch, or cutting operation.
Unfortunately, there are drawbacks associated with these different approaches. For example, although the gas flow reduction scheme of the '227 patent extends the lifetime of the electrode, to fully optimize the technology it is necessary to customize the length of hose between the on-off solenoid valve 33 and the torch 10 to achieve a proper volume and resulting gas ramp-down characteristics for a particular torch and consumable set (e.g., electrode and nozzle). This hose volume customization needs to be matched, e.g., to the specific closing characteristics of the on-off solenoid valve 33, such that a precise gas flow profile is achieved about the electrode 14 as the cut cycle is ended. More specifically, it was previously necessary to position on-off solenoid valves 33, 43 at a specified distance from the torch such as 12 inches, 4 feet, or 6 feet, depending upon the system being configured. Empirical determination of the pressure decay curve along with other mechanical adjustments and compensations were then performed to obtain a prolonged life of different consumable sets (e.g., electrodes and nozzles). Such tedious empirical determinations were performed for different torches, systems, consumable sets, and cutting conditions. In such systems, relocating the on-off solenoid valve by one foot, for example, from 4 feet away from the torch to 5 feet away from the torch without recalibrating the current ramp down rates resulted in a dramatic reduction in electrode life (on the order of 30%).
These control difficulties are exacerbated by the rapid system dynamics, which can all take place within a time span of about 300 milliseconds or less. The determination of the proper hose length and current ramp down characteristics to achieve an acceptable termination gas flow profile is empirically acquired and is extremely time consuming. Similar developmental problems are encountered when customizing the gas flow characteristics required for optimal use of the '494 patent, e.g., while transferring the current from the nozzle to the workpiece.
The chambered swirl ring of the '033 patent, while achieving an increase in electrode life, requires fabrication of a complex swirl ring design. Moreover, the inlet and outlet port diameters of such a swirl ring must be carefully fabricated to precise tolerances to achieve the desired gas flow characteristics. Although proper sizing of the swirl ring chamber volume and inlet and outlet port diameters can achieve the desired gas flow results, a given swirl ring is generally useful for only a certain torch type or consumable set necessitating the storage and availability of different swirl rings with varying design criteria. Performance of such swirl rings can also be adversely affected, e.g., by varying gas supply pressures and other gas flow parameters.
Finally, considering the gas distribution feed system of the '043 patent, the multiple flow channels for each gas stream are complex and require many component parts. What is needed is a less complicated, less expensive system to accomplish desired gas flow objectives.
What is also needed is a method and apparatus that can reliably accomplish all of these objectives using fewer component parts and at a reduced manufacturing cost.